Display device including pixel and electronic device

ABSTRACT

A display device capable of improving image quality is provided. The display device is provided with an adder circuit in and outside a display region, and the adder circuit has a function of adding a plurality of data supplied from a source driver together. Components of the adder circuit are divided and partly arranged in the display region. Thus, limitation on the size of the component included in the adder circuit can be eased, and data addition can be performed efficiently. In addition, by providing other components included in the adder circuit outside the display region, the number of wirings in the display region can be reduced and the aperture ratio of the pixel can be increased.

TECHNICAL FIELD

One embodiment of the present invention relates to a display device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. One embodiment of thepresent invention relates to a process, a machine, manufacture, or acomposition of matter. Accordingly, more specifically, the following canbe given as an example of the technical field of one embodiment of thepresent invention disclosed in this specification: a semiconductordevice, a display device, a liquid crystal display device, alight-emitting device, a lighting device, a power storage device, amemory device, an imaging device, a method for operating any of them, ora method for manufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a memory device, adisplay device, an imaging device, or an electronic device includes asemiconductor device.

BACKGROUND ART

A technique for forming transistors using metal oxide formed over asubstrate has been attracting attention. For example, a technique inwhich a transistor formed using zinc oxide or In—Ga—Zn-based oxide isused as a switching element or the like of a pixel of a display deviceis disclosed in Patent Document 1 and Patent Document 2.

Patent Document 3 discloses a memory device using a transistor with anextremely low off-state current in a memory cell.

Various improvements and applications of liquid crystal display deviceshave been attempted. For example, Patent Document 4 discloses atransparent display that performs display by an field sequentialoperation.

REFERENCES Patent Documents

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No. 2007-96055

[Patent Document 3] Japanese Published Patent Application No.2011-119674

[Patent Document 4] Japanese Published Patent Application No. 2018-21974

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The resolution of display devices has been increased; hardware capableof displaying images of an 8K4K (pixel number: 7680×4320) or higherresolution has been developed. In addition, the HDR (high dynamic range)display technique, which increases image quality by luminanceadjustment, has been introduced.

To perform clear gradation display, the width of data potentials thatcan be supplied to a display element is desired to be wide. Meanwhile,the output voltage of a source driver for a liquid crystal displaydevice is approximately 15 V, for example, and supplying a highervoltage to the display element requires a high output source driver tobe used. A high output source driver consumes high power, and requires anew driver IC to be developed in some cases.

Moreover, displaying a moving image more smoothly requires the framefrequency to be increased; however, since the increase in the number ofpixels shortens a horizontal period, increasing the frame frequency isdifficult. Achieving a structure where the frame frequency is easilyincreased makes the application to a field-sequential liquid crystaldisplay device or the like easier.

Solutions for such problems described above are expected; meanwhile, itis preferable to form a pixel circuit with a smaller number ofcomponents because increasing the constituent elements of the pixelcircuit reduces the aperture ratio.

Thus, an object of one embodiment of the present invention is to providea display device capable of improving image quality. Another object isto provide a display device capable of supplying voltage higher than orequal to the output voltage of a source driver to a display element.Another object is to provide a display device capable of enhancing theluminance of a displayed image. Another object is to provide a displaydevice capable of increasing the frame frequency. Another object is toprovide a display device in which the aperture ratio of a pixel can beincreased.

Another object is to provide a display device with low powerconsumption. Another object is to provide a highly reliable displaydevice. Another object is to provide a novel display device or the like.Another object is to provide a method for driving any of the abovedisplay devices. Another object is to provide a novel semiconductordevice or the like.

Note that the description of these objects does not preclude theexistence of other objects. One embodiment of the present invention doesnot have to achieve all these objects. Other objects are apparent fromthe description of the specification, the drawings, the claims, and thelike, and other objects can be derived from the description of thespecification, the drawings, the claims, and the like.

Means for Solving the Problems

One embodiment of the present invention relates to a display devicecapable of improving image quality.

One embodiment of the present invention is a display device including apixel and a first circuit. The pixel and the first circuit areelectrically connected to each other. The first circuit includes atransistor and a first capacitor. The transistor is provided outside adisplay region. The first capacitor is provided in the display region.The first circuit has a function of adding first data and second data togenerate third data. The pixel has a function of retaining the thirddata and a function of performing display in accordance with the thirddata.

The first capacitor includes a plurality of second capacitors. Theplurality of second capacitors are connected in parallel.

The first circuit can include a first transistor, a second transistor, athird transistor, and the first capacitor. One of a source and a drainof the first transistor can be electrically connected to the pixel. Theone of the source and the drain of the first transistor can beelectrically connected to one electrode of the first capacitor. Theother electrode of the first capacitor can be electrically connected toone of a source and a drain of the second transistor. The one of thesource and the drain of the second transistor can be electricallyconnected to one of a source and a drain of the third transistor. Theother of the source and the drain of the first transistor can beelectrically connected to the other of the source and the drain of thesecond transistor.

The pixel can include a fourth transistor and a second circuit. One of asource and a drain of the fourth transistor can be electricallyconnected to the first circuit. The other of the source and the drain ofthe fourth transistor can be electrically connected to the secondcircuit. The second circuit can include a display element.

The second circuit can include a fifth transistor, a third capacitor,and a light-emitting element as the display element. A gate of the fifthtransistor can be electrically connected to the other of the source andthe drain of the fourth transistor. One of a source and a drain of thefifth transistor can be electrically connected to one electrode of thelight-emitting element. The one electrode of the light-emitting elementcan be electrically connected to one electrode of the third capacitor.The other electrode of the third capacitor can be electrically connectedto the gate of the fifth transistor.

The second circuit may further include a sixth transistor. One of asource and a drain of the sixth transistor may be electrically connectedto the one electrode of the light-emitting element. The other of thesource and the drain of the sixth transistor may be electricallyconnected to the one of the source and the drain of the fifthtransistor.

Alternatively, the second circuit may include a liquid crystal elementas the display element. One electrode of the liquid crystal element maybe electrically connected to the one of the source and the drain of thefourth transistor. The second circuit may further include a fourthcapacitor. One electrode of the fourth capacitor may be electricallyconnected the one electrode of the liquid crystal element.

It is preferable that the transistors included in the first circuit andthe pixel each contain a metal oxide in a channel formation region, andthat the metal oxide include In, Zn, and M (M is Al, Ti, Ga, Sn, Y, Zr,La, Ce, Nd, or Hf).

The channel width of the transistor included in the first circuit ispreferably larger than the channel width of the transistor included inthe pixel.

Effect of the Invention

With the use of one embodiment of the present invention, a displaydevice capable of improving image quality can be provided.Alternatively, a display device capable of supplying voltage higher thanor equal to the output voltage of a source driver to a display elementcan be provided. Alternatively, a display device capable of enhancingthe luminance of a displayed image can be provided. Alternatively, adisplay device capable of increasing the frame frequency can beprovided. Alternatively, a display device in which the aperture ratio ofa pixel can be increased can be provided.

Alternatively, a display device with low power consumption can beprovided. Alternatively, a highly reliable display device can beprovided. Alternatively, a novel display device or the like can beprovided. Alternatively, a method for driving any of the above displaydevices can be provided. Alternatively, a novel semiconductor device orthe like can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A diagram illustrating a display device.

FIG. 2 A diagram illustrating an adder circuit and pixels.

FIGS. 3 (A) and (B) Diagrams illustrating display devices.

FIG. 4 A timing chart showing operations of an adder circuit and pixels.

FIG. 5 A diagram illustrating an adder circuit and pixels.

FIG. 6 (A) to (D) Diagrams each illustrating a circuit block.

FIG. 7 (A) to (D) Diagrams each illustrating a circuit block.

FIG. 8 (A) to (C) Diagrams each illustrating a circuit block.

FIGS. 9 (A) and (B) Diagrams each illustrating an adder circuit and apixel.

FIG. 10 A diagram illustrating structures of an adder circuit and pixelsused for simulation.

FIG. 11 A timing chart used for simulation.

FIG. 12 (A) to (D) Diagrams showing simulation results.

FIG. 13 A diagram illustrating a pixel layout.

FIG. 14 (A) to (C) Diagrams each illustrating a display device.

FIGS. 15 (A) and (B) Diagrams illustrating a touch panel.

FIGS. 16 (A) and (B) Diagrams each illustrating a display device.

FIG. 17 A diagram illustrating a display device.

FIGS. 18 (A) and (B) Diagrams each illustrating a display device.

FIGS. 19 (A) and (B) Diagrams each illustrating a display device.

FIG. 20 (A) to (E) Diagrams each illustrating a display device.

FIG. 21 (A1) to (C2) Diagrams illustrating transistors.

FIG. 22 (A1) to (C2) Diagrams illustrating transistors.

FIG. 23 (A1) to (C2) Diagrams illustrating transistors.

FIG. 24 (A1) to (C2) Diagrams illustrating transistors.

FIG. 25 (A) to (F) Diagrams illustrating electronic devices.

MODE FOR CARRYING OUT THE INVENTION

Embodiments are described in detail with reference to the drawings.However, the present invention is not limited to the followingdescription, and it is readily appreciated by those skilled in the artthat modes and details can be modified in various ways without departingfrom the spirit and the scope of the present invention. Therefore, thepresent invention should not be interpreted as being limited to thedescriptions of embodiments below. Note that in structures of theinvention described below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and the description thereof is not repeated in some cases. Thesame components are denoted by different hatching patterns in differentdrawings, or the hatching patterns are omitted in some cases.

Even in the case where a single component is illustrated in a circuitdiagram, the component may be composed of a plurality of parts as longas there is no functional inconvenience. For example, in some cases, aplurality of transistors that operate as a switch are connected inseries or in parallel. In some cases, capacitors are divided andarranged in a plurality of positions.

One conductor has a plurality of functions such as a wiring, anelectrode, and a terminal in some cases. In this specification, aplurality of names are used for the same component in some cases. Evenin the case where elements are illustrated in a circuit diagram as ifthey were directly connected to each other, the elements may actually beconnected to each other through a plurality of conductors; in thisspecification, even such a configuration is included in directconnection.

Embodiment 1

In this embodiment, a display device that is one embodiment of thepresent invention will be described with reference to drawings.

One embodiment of the present invention is a display device in which acircuit having a function of adding data (hereinafter referred to as anadder circuit) is provided in and outside a display region. The addercircuit has a function of adding data supplied from a source driver.Therefore, a voltage higher than or equal to the output of the sourcedriver can be generated.

The adder circuit is electrically connected to all pixels arranged in acolumn direction in the display region, and components thereof aredivided and partly arranged in the display region. Thus, limitation onthe size of the component included in the adder circuit can be eased,and data addition can be performed efficiently. In addition, byproviding other components included in the adder circuit outside thedisplay region, the number of wirings in the display region can bereduced and the aperture ratio of the pixel can be increased.

FIG. 1 is a diagram illustrating a display device of one embodiment ofthe present invention. The display device includes pixels 10 arranged ina column direction and in a row direction, a source driver 12, a gatedriver 13, and circuits 11. The source driver 12 is electricallyconnected to the circuits 11. The gate driver 13 is electricallyconnected to the pixels 10. The circuits 11 are electrically connectedto the pixels 10. Although an example is shown in which one gate driver13 is provided on one end side of a display region 15, another gatedriver 13 may be additionally provided on the opposite end side and thepixels 10 may be driven by the two gate drivers.

The circuit 11 can be provided for each column and is electricallyconnected to all the pixels 10 arranged in the same column. Componentsof the circuit 11 can be provided in and outside the display region.

The circuit 11 is an adder circuit, and has a function of adding firstdata and second data supplied from the source driver 12 by capacitivecoupling to generate third data. The pixel 10 includes a transistor anda display element and has a function of retaining the third data and afunction of performing display with the display element in accordancewith the third data.

FIG. 2 is a diagram illustrating the circuit 11 and the pixels 10 (thepixels 10[m, 1] to 10[m, n] (m and n are each a natural number greaterthan or equal to 1)) that are arranged in a given column (the m-thcolumn) of the display device illustrated in FIG. 1.

The circuit 11 can have a structure including a transistor 101, atransistor 102, a transistor 103, and a capacitor 104. One of a sourceand a drain of the transistor 101 is electrically connected to oneelectrode of the capacitor 104. The other electrode of the capacitor 104is electrically connected to one of a source and a drain of thetransistor 102. The one of the source and the drain of the transistor102 is electrically connected to one of a source and a drain of thetransistor 103.

Here, the capacitor 104 is consisted of a plurality of capacitors 106connected in parallel. When the capacitors 106 are separately providedin the display region, the total area of the capacitor 104 can be easilyincreased. Furthermore, the area occupied by the circuit 11 outside thedisplay region can be reduced, and thus the frame can be narrowed. Notethat one or more of the capacitors 106 may be provided outside thedisplay region. The number of capacitors 106 is not necessarily the sameas the number of the pixels 10. By adjusting the number of capacitors106 connected in parallel, the capacitor 104 can have a desiredcapacitance value.

The capacitor 106 includes a wiring 125 as one electrode and anotherwiring overlapping with the wiring 125 as the other electrode. Thus,even when the capacitor 106 is provided in the display region asillustrated in FIG. 2, the aperture ratio is not significantlydecreased.

The pixel 10 can have a structure including a transistor 105 and acircuit block 110. The circuit block 110 can have a structure includinga transistor, a capacitor, a display element, and the like. One of asource and a drain of the transistor 105 is electrically connected tothe one of the source and the drain of the transistor 101. The other ofthe source and the drain of the transistor 105 is electrically connectedto the circuit block 110.

Here, a wiring (the wiring 125) that connects the one of the source andthe drain of the transistor 101, the one electrode of the capacitor 104,and the one of the source and the drain of the transistor 105 isreferred to as a node NM. A wiring that connects the other of the sourceand the drain of the transistor 105 and the circuit block 110 isreferred to as a node NP. The node NP can be floating, and the displayelement included in the circuit block 110 operates in accordance withthe potential of the node NP.

Connections between the components of the circuit 11 and the pixels 10and a variety of wirings are described. A gate of the transistor 101 iselectrically connected to a wiring 121. A gate of the transistor 102 iselectrically connected to a wiring 122. A gate of the transistor 103 iselectrically connected to the wiring 121. A gate of the transistor 105is electrically connected to a wiring 123. The one of the source and thedrain of the transistor 101 is electrically connected to the wiring 125.The other of the source and the drain of the transistor 102 and theother of the source and the drain of the transistor 101 are electricallyconnected to a wiring 124. The other of the source and the drain of thetransistor 103 is electrically connected to a wiring capable ofsupplying V_(ref) (e.g., a reference potential such as 0 V).

The wirings 121, 122, and 123 (123[1] to 123[n]) each have a function ofa gate line. For example, the wirings 121 and 122 can be electricallyconnected to a circuit for controlling an operation of the circuit 11.The wiring 123 can be electrically connected to the gate driver 13. Thewiring 124 can be electrically connected to the source driver 12 (seeFIG. 1).

In the circuit 11, first, first data (weight: W) is written to the nodeNM. At this time, “V_(ref)” is supplied to the other electrode of thecapacitor 104, and the capacitor 104 is made to retain “W−V_(ref)”.Next, the node NM is set to be floating and second data (data: D) issupplied to the other electrode of the capacitor 104.

At this time, when the capacitance value of the capacitor 104 is C₁₀₄and the capacitance value of the node NM is C_(NM), the potential of thenode NM is W+(C₁₀₄/(C₁₀₄+C_(NM)))×(D−V_(ref)). Here, when the value ofC₁₀₄ is made large and the value of C_(NM) becomes negligible, thepotential of the node NM becomes “W+D−V_(ref) ^(”). In one embodiment ofthe present invention, it is easy to increase the total area of thecapacitor 104 to increase the capacitance value as described above, sothat data addition can be performed efficiently.

Therefore, when “W”=“D”, “V_(ref)”=0 V, and C₁₀₄ is sufficiently higherthan C_(NM), the potential of the node NM becomes closer to “2D” or “2W”, so that a potential that is approximately twice the output of thesource driver 12 can be supplied to the node NM.

By such an action, a high voltage can be generated even using ageneral-purpose driver IC. For example, a liquid crystal element thatrequires a high voltage for gray level control can be driven.Alternatively, since a voltage that is supplied from the source driver12 in order to drive a general liquid crystal element or light-emittingelement can be approximately halved, the power consumption of thedisplay device can be reduced.

Furthermore, correction data may be supplied as the first data (weight:W). For example, when luminance correction data is added to image data,variation in the luminance unique to the display device can becorrected. Alternatively, correction data may be used for HDR displaybecause the luminance can be corrected pixel by pixel. In the case wherea light-emitting element is used as the display element, the displayquality is affected by variation in the threshold voltage of a drivingtransistor; thus, the display quality may be improved by supplying, asthe first data (weight: W), threshold-voltage correction data for thetransistor.

Note that the first data (weight: W) and the second data (data: D) maybe interchanged.

In one embodiment of the present invention, the transistor 105 of aspecific pixel 10 is turned on in accordance with the above-describedoperation of adding the potentials, and the potential of the node NP(=the potential of the node NM) is determined. Such an operation isperformed in the pixel 10[m, 1] to the pixel 10[m, n] successively,whereby the potential of the node NP of each of the pixels 10 can bedetermined. That is, different image data can be supplied to therespective pixels 10.

The node NM and the node NP function as storage nodes. The transistorconnected to the corresponding node is turned on, whereby data can bewritten to the node. The transistor is turned off, whereby the data canbe retained in the node. The use of a transistor with an extremely lowoff-state current as the transistor enables leakage current to bereduced and the potential of the node to be retained for a long time. Asthe transistor, a transistor using a metal oxide in a channel formationregion (hereinafter referred to as an OS transistor) can be used, forexample.

Specifically, OS transistors are preferably used as any or all of thetransistors 101, 102, 103, and 105. An OS transistor may also be usedfor any of the components included in the circuit block 110. In the caseof operating within a range where the amount of leakage current isacceptable, a transistor including Si in a channel formation region(hereinafter, Si transistor) may be used. Alternatively, an OStransistor and a Si transistor may be used together. Examples of the Sitransistor include a transistor including amorphous silicon and atransistor including crystalline silicon (typically, low-temperaturepolysilicon or single crystal silicon).

As a semiconductor material used for an OS transistor, a metal oxidewhose energy gap is greater than or equal to 2 eV, preferably greaterthan or equal to 2.5 eV, further preferably greater than or equal to 3eV can be used. A typical example is an oxide semiconductor containingindium, and a CAAC-OS or a CAC-OS described later can be used, forexample. A CAAC-OS has a crystal structure including stable atoms and issuitable for a transistor that is required to have high reliability, andthe like. A CAC-OS has high mobility and is suitable for a transistorthat operates at high speed, and the like.

In an OS transistor, a semiconductor layer has a large energy gap, andthus the OS transistor has an extremely low off-state current of severalyA/μm (current per micrometer of a channel width). An OS transistor hasfeatures such that impact ionization, an avalanche breakdown, ashort-channel effect, or the like does not occur, which are differentfrom those of a Si transistor. Thus, the use of an OS transistor enablesformation of a highly reliable circuit. Moreover, variations inelectrical characteristics due to crystallinity unevenness, which arecaused in Si transistors, are less likely to occur in OS transistors.

A semiconductor layer included in the OS transistor can be, for example,a film represented by an In-M-Zn-based oxide that contains indium, zinc,and M (a metal such as aluminum, titanium, gallium, germanium, yttrium,zirconium, lanthanum, cerium, tin, neodymium, or hafnium).

In the case where the oxide semiconductor contained in the semiconductorlayer is an In-M-Zn-based oxide, it is preferable that the atomic ratioof metal elements in a sputtering target used to form a film of theIn-M-Zn oxide satisfy In M and Zn M. The atomic ratio of metal elementsin such a sputtering target is preferably, for example, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=4:2:3, In:M:Zn=4:2:4.1,In:M:Zn=5:1:6, In:M:Zn=5:1:7, or In:M:Zn=5:1:8. Note that the atomicratio in the formed semiconductor layer varies from the above atomicratio of metal elements in the sputtering target in a range of ±40%.

An oxide semiconductor with low carrier density is used for thesemiconductor layer.

For example, the semiconductor layer may use an oxide semiconductorwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, still further preferably lower than or equal to1×10¹¹/cm³, even further preferably lower than 1×10¹⁰/cm³, and higherthan or equal to 1×10⁻⁹/cm³. Such an oxide semiconductor is referred toas a highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor. The oxide semiconductor has a low densityof defect states and can thus be regarded as having stablecharacteristics.

Note that, without limitation to these, a material with an appropriatecomposition may be used in accordance with required semiconductorcharacteristics and electrical characteristics (e.g., field-effectmobility and threshold voltage) of the transistor. To obtain therequired semiconductor characteristics of the transistor, it ispreferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio between a metal element and oxygen, theinteratomic distance, the density, and the like of the semiconductorlayer be set to appropriate values.

When the oxide semiconductor in the semiconductor layer contains siliconor carbon, which is an element belonging to Group 14, the amount ofoxygen vacancies is increased in the semiconductor layer, and thesemiconductor layer becomes n-type. Thus, the concentration of siliconor carbon (the concentration obtained by SIMS: Secondary Ion MassSpectrometry) in the semiconductor layer is set to 2×10¹⁸ atoms/cm³ orlower, preferably 2×10¹⁷ atoms/cm³ or lower.

Alkali metal and alkaline earth metal might generate carriers whenbonded to an oxide semiconductor, in which case the off-state current ofthe transistor might be increased. Therefore, the concentration ofalkali metal or alkaline earth metal in the semiconductor layer (theconcentration obtained by SIMS) is set to 1×10¹⁸ atoms/cm³ or lower,preferably 2×10¹⁶ atoms/cm³ or lower.

When the oxide semiconductor in the semiconductor layer containsnitrogen, electrons functioning as carriers are generated and thecarrier density increases, so that the semiconductor layer easilybecomes n-type. Thus, a transistor using an oxide semiconductor thatcontains nitrogen is likely to be normally on. Hence, the concentrationof nitrogen in the semiconductor layer (the concentration obtained bySIMS) is preferably set to 5×10¹⁸ atoms/cm³ or lower.

When hydrogen is contained in an oxide semiconductor included in thesemiconductor layer, hydrogen reacts with oxygen bonded to a metal atomto be water, and thus sometimes causes an oxygen vacancy in the oxidesemiconductor. If the channel formation region in the oxidesemiconductor includes oxygen vacancies, the transistor sometimes hasnormally-on characteristics. In some cases, a defect that is an oxygenvacancy into which hydrogen enters functions as a donor and generates anelectron serving as a carrier. In other cases, bonding of part ofhydrogen to oxygen bonded to a metal atom generates electrons serving ascarriers. Thus, a transistor including an oxide semiconductor thatcontains a large amount of hydrogen is likely to have normally-oncharacteristics.

A defect in which hydrogen has entered an oxygen vacancy can function asa donor of the oxide semiconductor. However, it is difficult to evaluatethe defects quantitatively. Thus, the oxide semiconductor is sometimesevaluated by not its donor concentration but its carrier concentration.Therefore, in this specification and the like, the carrier concentrationassuming the state where an electric field is not applied is sometimesused, instead of the donor concentration, as the parameter of the oxidesemiconductor. That is, “carrier concentration” in this specificationand the like can be replaced with “donor concentration” in some cases.

Therefore, hydrogen in the oxide semiconductor is preferably reduced asmuch as possible. Specifically, the hydrogen concentration in the oxidesemiconductor obtained by SIMS is lower than 1×10²⁰ atoms/cm³,preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than5×10¹⁸ atoms/cm³, still further preferably lower than 1×10¹⁸ atoms/cm³.When an oxide semiconductor with a sufficiently low concentration ofimpurities such as hydrogen is used for a channel formation region of atransistor, the transistor can have stable electrical characteristics.

The semiconductor layer may have a non-single-crystal structure, forexample. Examples of a non-single-crystal structure include a CAAC-OS(C-Axis Aligned Crystalline Oxide Semiconductor) including a c-axisaligned crystal, a polycrystalline structure, a microcrystallinestructure, and an amorphous structure. Among the non-single-crystalstructures, an amorphous structure has the highest density of defectstates, whereas the CAAC-OS has the lowest density of defect states.

An oxide semiconductor film having an amorphous structure has disorderedatomic arrangement and no crystalline component, for example. In anotherexample, an oxide film having an amorphous structure has a completelyamorphous structure and no crystal part.

Note that the semiconductor layer may be a mixed film including two ormore of the following: a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a region of CAAC-OS, and a region having a single crystalstructure. The mixed film has, for example, a single-layer structure ora layered structure including two or more of the foregoing regions insome cases.

The composition of a CAC (Cloud-Aligned Composite)-OS, which is oneembodiment of a non-single-crystal semiconductor layer, is describedbelow.

The CAC-OS has, for example, a composition in which elements containedin an oxide semiconductor are unevenly distributed. Materials containingunevenly distributed elements each have a size of greater than or equalto 0.5 nm and less than or equal to 10 nm, preferably greater than orequal to 1 nm and less than or equal to 2 nm, or a similar size. Notethat in the following description of an oxide semiconductor, a state inwhich one or more metal elements are unevenly distributed and regionscontaining the metal element(s) are mixed is referred to as a mosaicpattern or a patch-like pattern. The region has a size greater than orequal to 0.5 nm and less than or equal to 10 nm, preferably greater thanor equal to 1 nm and less than or equal to 2 nm, or a similar size.

Note that an oxide semiconductor preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, oneor more of aluminum, gallium, yttrium, copper, vanadium, beryllium,boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like may be contained.

For example, of the CAC-OS, an In-Ga—Zn oxide with the CAC composition(such an In-Ga—Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite oxide semiconductor with acomposition in which a region containing GaO_(X3) as a main componentand a region containing In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent are mixed. Note that in this specification, when the atomicratio of In to an element M in a first region is greater than the atomicratio of In to an element M in a second region, for example, the firstregion is described as having higher In concentration than the secondregion.

Note that a compound containing In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals have c-axis alignment and are connected in the a-b planedirection without alignment.

The CAC-OS relates to the material composition of an oxidesemiconductor. In a material composition of a CAC-OS containing In, Ga,Zn, and O, nanoparticle regions containing Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions containing In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Thus, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a layered structure including two or more filmswith different atomic ratios is not included. For example, a two-layerstructure of a film containing In as a main component and a filmcontaining Ga as a main component is not included.

A boundary between the region containing GaO_(X3) as a main componentand the region containing In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

Note that in the case where one kind or a plurality of kinds selectedfrom aluminum, yttrium, copper, vanadium, beryllium, boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the likeare contained instead of gallium, the CAC-OS refers to a composition inwhich some regions that include the metal element(s) as a main componentand are observed as nanoparticles and some regions that include In as amain component and are observed as nanoparticles are randomly dispersedin a mosaic pattern.

The CAC-OS can be formed by a sputtering method under a condition wherea substrate is not heated intentionally, for example. In the case wherethe CAC-OS is formed by a sputtering method, one or more of an inert gas(typically, argon), an oxygen gas, and a nitrogen gas may be used as adeposition gas. The flow rate of the oxygen gas to the total flow rateof the deposition gas in deposition is preferably as low as possible;for example, the flow rate of the oxygen gas is higher than or equal to0% and lower than 30%, preferably higher than or equal to 0% and lowerthan or equal to 10%.

The CAC-OS is characterized in that a clear peak is not observed whenmeasurement is conducted using a θ/2θ scan by an out-of-plane method,which is an X-ray diffraction (XRD) measurement method. That is, it isfound by the XRD measurement that there are no alignment in the a-bplane direction and no alignment in the c-axis direction in the measuredareas.

In an electron diffraction pattern of the CAC-OS that is obtained byirradiation with an electron beam with a probe diameter of 1 nm (alsoreferred to as a nanometer-sized electron beam), a ring-like region(ring region) with high luminance and a plurality of bright spots in thering region are observed. Thus, it is found from the electrondiffraction pattern that the crystal structure of the CAC-OS includes annc (nano-crystal) structure that does not show alignment in the planedirection and the cross-sectional direction.

For example, energy dispersive X-ray spectroscopy (EDX) is used toobtain EDX mapping, and according to the EDX mapping, the CAC-OS of theIn-Ga—Zn oxide has a composition in which the region containing GaO_(X3)as a main component and the region containing In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component are unevenly distributed and mixed.

The CAC-OS has a structure different from that of an IGZO compound inwhich metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, in the CAC-OS, theregion containing GaO_(X3) or the like as a main component and theregion containing In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main componentare separated to form a mosaic pattern.

The conductivity of the region containing In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component is higher than that of the regioncontaining GaO_(X3) or the like as a main component. In other words,when carriers flow through the region containing In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component, the conductivity of an oxide semiconductoris generated. Accordingly, when the regions containingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component are distributedlike a cloud in an oxide semiconductor, high field-effect mobility (μ)can be achieved.

By contrast, the insulating property of the region containing GaO_(X3)or the like as a main component is superior to that of the regioncontaining In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. Inother words, when the regions containing GaO_(X3) or the like as a maincomponent are distributed in an oxide semiconductor, leakage current canbe suppressed and favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used in a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complementeach other, whereby high on-state current (Ion) and high field-effectmobility (μ) can be achieved.

A semiconductor element using a CAC-OS has high reliability. Thus, theCAC-OS is suitably used as a material in a variety of semiconductordevices.

In the display device of one embodiment of the present invention, acomponent other than the capacitor 104 of the circuit 11 may beincorporated in the source driver 12 as illustrated in FIG. 3(A). Thisstructure enables a narrow frame.

When the pixel circuits and the source driver 12 are monolithicallyformed over a substrate, a stack structure including a region where thesource driver 12 and any of the components of the circuit 11 overlapwith each other may be employed. This structure can increase thecomponent design flexibility and electrical characteristics of thecircuit 11.

Although FIG. 1 illustrates the example in which the circuit 11 isprovided for each column, as illustrated in FIG. 3(B), a selectioncircuit 16 may be provided between the circuit 11 and the pixels 10 andone circuit 11 may perform data writing on a plurality of columns. Withsuch a structure, the number of circuits 11 can be reduced, and theframe can be narrowed. Although FIG. 3(B) illustrates an example inwhich a combination of one circuit 11 and one selection circuit 16performs data writing on pixels in three columns, it is not limitedthereto and the number of columns may be determined in the range ofallowable writing time.

Since the transistors 101, 102, and 103 of the circuit 11 are providedoutside the display region, the size is less likely to be limited andthe channel width thereof can be larger than that of the transistorprovided in the pixel. The use of the transistor with a large channelwidth makes it possible to shorten the charge and discharge time for thewiring 125 and the like and to facilitate an increase in the framefrequency. Furthermore, such a transistor can be easily used in ahigh-resolution display with a short horizontal period.

When OS transistors are used as the transistors 101, 102, and 103, thecircuit 11 can withstand a high voltage, and a stable operation can beperformed even when a voltage generated in data addition is several tensof voltage. In the case where the transistors 101, 102, and 103 are Sitransistors provided in an IC chip, higher-speed operation can beperformed. In the case where the transistors 101, 102, and 103 areprovided in an IC chip, the transistors may be OS transistors.

Next, a method for writing data to the pixel 10 using the circuit 11 isdescribed using a timing chart in FIG. 4. Note that in the followingdescription, a high potential is represented by “H” and a low potentialis represented by “L”. In addition, the weight supplied to the pixel10[1] is “W[1]”, image data supplied to the pixel 10[1] is “D[1]”, theweight supplied to the pixel 10[2] is “W[2]”, and image data supplied tothe pixel 10[2] is “D[2]”. As “V_(ref)”, 0 V, a GND potential, or acertain reference potential can be used, for example.

Note that in potential distribution, potential coupling, or potentialloss, detailed changes due to a circuit configuration, operation timing,or the like are not considered. In addition, a potential change due tocapacitive coupling depends on the ratio of the capacitance of apotential supplier to that of a potential receiver; however, for clarityof description, the capacitance value of the circuit block 110 isassumed to be sufficiently small.

At time T1, “W[1]” is supplied to the wiring 124 and the potentials ofthe wirings 121 and 123[1] are set to “H”, whereby the transistor 103 isturned on, and the potential of the other electrode of the capacitor 104becomes “V_(ref)”. The operation is a reset operation for an additionoperation (capacitive coupling operation) to be performed later.

Furthermore, the transistors 101 and 105 are turned on, and thepotential of the wiring 124 is written to the node NP[1]. This operationis an operation of writing weight, and the potential of the node NP[1]becomes “W[1]”.

At time T2, the potential of the wiring 121 is set to “L” and thepotential of the wiring 123[1] is set to “H”, whereby the transistors101 and 103 are turned off. At this time, “W[1]” is retained in the nodeNP[1]. In addition, “W[1]−V_(ref)” is retained in the capacitor 104. Theoperation of writing “W[1]” in the pixel 10[1] has been described sofar.

At time T3, “D[1]” is supplied to the wiring 124, the potential of thewiring 121 is set to “L”, and the potentials of the wirings 122 and123[1] are set “H”, whereby the transistors 102 and 105 are turned on.At this time, the potential of the other electrode of the capacitor 104becomes “D[1]”, and “D[1]” is added to the potential of the node NP[1]owing to capacitive coupling. This operation is the addition operation,and the potential of the node NP[1] becomes “W[1]+D[1]−V_(ref)”. At thistime, when “V_(ref)”=0, the potential of the node NP[1] becomes“W[1]+D[1]”. The potential of the node NP[1] is supplied to the displayelement, and display is performed.

At time T4, the potentials of the wirings 121, 122, and 123[1] are setto “L”, whereby the transistor 105 is turned off, the potential of thenode NP[1] is retained, and the display is kept until an operation ofthe next frame. The above is the description of the operations of thepixel 10[1].

By applying the same operations to the pixel 10[2] from Time T5 to TimeT8, display in accordance with “W[2]+D[2]” can be performed in the pixel10[2].

The circuit 11 may be provided not only on one end side of the displayregion 15 but also on the opposite end side, as illustrated in FIG. 5.

Here, the circuit 11 provided on one end side of the display region 15is a circuit 11 a. An operation of the circuit 11 a is controlled bysignals supplied from wirings 121 a and 122 a. The circuit 11 a iselectrically connected to a source driver 12 a. The circuit 11 providedon the opposite end side of the display region 15 is a circuit 11 b. Anoperation of the circuit 11 b is controlled by signals supplied fromwirings 121 b and 122 b. The circuit 11 b is electrically connected to asource driver 12 b.

The circuit 11 a and the circuit 11 b operate to output the same data atthe same timing. That is, the same data is output from the sourcedrivers 12 a and 12 b at the same timing, and the same operation signalsare supplied to the wiring 121 a and the wiring 121 b, and the wiring121 a and the wiring 121 b at the same timing.

By such an operation, the circuit 11 a and the circuit 11 b can operateat the same time and the same data can be output to the wiring 125.Thus, the wiring 125 can be charged and discharged at high speed, and adisplay device including a large number of pixels with a shorthorizontal period, a large-sized display device including the wiring 125with high parasitic capacitance, or the like can be easily achieved.

FIGS. 6(A) to 6(C) each illustrate a structure example including alight-emitting element as the display element, which can be applied tothe circuit block 110.

The structure illustrated in FIG. 6(A) includes a transistor 111, acapacitor 113, and a light-emitting element 114. One of a source and adrain of the transistor 111 is electrically connected to one electrodeof the light-emitting element 114. The one electrode of thelight-emitting element 114 is electrically connected to one electrode ofthe capacitor 113. The other electrode of the capacitor 113 iselectrically connected to a gate of the transistor 111. The gate of thetransistor 111 is electrically connected to the node NP.

The other of the source and the drain of the transistor 111 iselectrically connected to a wiring 128. The other electrode of thelight-emitting element 114 is electrically connected to a wiring 129.The wirings 128 and 129 have a function of supplying power. For example,the wiring 128 is capable of supplying a high potential power. Thewiring 129 is capable of supplying a low potential power.

In the structure illustrated in FIG. 6(A), current flows through thelight-emitting element 114 when the potential of the node NM becomeshigher than or equal to the threshold voltage of the transistor 111.Therefore, light emission of the light-emitting element 114 is startedin some cases at the time when the weight (W) is written to the node NP,and the application might be limited.

Alternatively, as illustrated in FIG. 6(B), the one electrode of thelight-emitting element 114 may be electrically connected to the wiring128, and the other electrode of the light-emitting element 114 may beelectrically connected to the other of the source and the drain of thetransistor 111. The structure can also be applied to other circuitsblocks 110 including light-emitting elements 114.

FIG. 6(C) is a structure in which a transistor 112 is added to thestructure of FIG. 6(A). One of a source and a drain of the transistor112 is electrically connected to the one of the source and the drain ofthe transistor 111. The other of the source and the drain of thetransistor 112 is electrically connected to the light-emitting element114. A gate of the transistor 112 is electrically connected to a wiring130. The wiring 130 can have a function of a signal line that controlsthe conduction of the transistor 112.

In the structure, current flows through the light-emitting element 114when the potential of the node NP is higher than or equal to thethreshold voltage of the transistor 111 and the transistor 112 is turnedon. Thus, light emission of the light-emitting element 114 can bestarted at any time after the operation of adding the weight (W) and thedata (D).

FIG. 6(D) is a structure in which a transistor 115 is added to thestructure of FIG. 6(C). One of a source and a drain of the transistor115 is electrically connected to the one of the source and the drain ofthe transistor 111. The other of the source and the drain of thetransistor 115 is electrically connected to a wiring 131. A gate of thetransistor 115 is electrically connected to a wiring 132. The wiring 132can have a function of a signal line that controls the conduction of thetransistor 115.

The wiring 131 can be electrically connected to a supply source of acertain potential such as a reference potential. The certain potentialis supplied from the wiring 131 to the one of the source and the drainof the transistor 111, whereby write of image data can be stable.

In addition, the wiring 131 can be connected to a circuit 120 and canalso have a function of a monitor line. The circuit 120 can have one ormore of a function of the supply source of a certain potential, afunction of obtaining electric characteristics of the transistor 111,and a function of generating correction data.

FIGS. 7(A) to 7(D) each illustrate an example of a structure including aliquid crystal element as the display element, which can be applied tothe circuit block 110.

The structure illustrated in FIG. 7(A) includes a capacitor 116 and aliquid crystal element 117. One electrode of the liquid crystal element117 is electrically connected to one electrode of the capacitor 116. Theone electrode of the capacitor 116 is electrically connected to the nodeNP.

The other electrode of the capacitor 116 is electrically connected to awiring 133. The other electrode of the liquid crystal element 117 iselectrically connected to a wiring 134. The wirings 133 and 134 have afunction of supplying power. The wirings 133 and 134 are capable ofsupplying a reference potential such as GND or 0 V or a given potential,for example.

Note that the capacitor 116 may be omitted as illustrated in FIG. 7(B).As described above, an OS transistor can be used as the transistorconnected to the node NP. Since an OS transistor has extremely lowleakage current, display can be kept for a comparatively long time evenwhen the capacitor 116 functioning as a storage capacitor is omitted. Inaddition, omitting the capacitor 116 is effective not only in the caseof such a structure of the transistor but also in the case wherehigh-speed operation allows a shorter period for displaying an image asin the field-sequential driving. The aperture ratio can be improved byomitting the capacitor 116. Alternatively, the transmittance of thepixel can be improved.

In the structures of FIGS. 7(A) and 7(B), the operation of the liquidcrystal element 117 is started when the potential of the node NP isdetermined to be higher than or equal to the operation threshold of theliquid crystal element 117. Thus, an display operation is started insome cases at the time when the weight is written to the node NP, andthe application might be limited. In a transmissive liquid crystaldisplay device, however, an unnecessary display operation can be madeless visible when the operation of, for example, turning off a backlightuntil the operation of adding the weight (W) and the data (D) isfinished is also performed.

FIG. 7(C) illustrates a structure in which a transistor 118 is added tothe structure of FIG. 7(A). One of a source and a drain of thetransistor 118 is electrically connected to the one electrode of thecapacitor 116. The other of the source and the drain of the transistor118 is electrically connected to the node NP. A gate of the transistor118 is electrically connected to a wiring 127. The wiring 127 can have afunction of a signal line that controls the conduction of the transistor118.

In the structure, the potential of the node NP is applied to the liquidcrystal element 117 when the transistor 118 is turned on. Thus, theoperation of the liquid crystal element can be started at any time afterthe operation of adding the weight (W) and the data (D).

While the transistor 118 is in a non-conduction state, the potentialssupplied to the capacitor 116 and the liquid crystal element 117 areretained continuously; thus, the potentials supplied to the capacitor116 and the liquid crystal element 117 are preferably reset before theimage data is rewritten. For the reset, a reset potential is supplied tothe wiring 124 to bring the transistor 101 and the transistor 118 intoconduction at the same time, for example.

FIG. 7(D) illustrates a structure in which a transistor 119 is added tothe structure of FIG. 7(C). One of a source and a drain of thetransistor 119 is electrically connected to the one electrode of theliquid crystal element 117. The other of the source and the drain of thetransistor 119 is electrically connected to the wiring 131. A gate ofthe transistor 119 is electrically connected to a wiring 138. The wiring138 can have a function of a signal line that controls the conduction ofthe transistor 119.

The circuit 120 electrically connected to the wiring 131 is as describedabove using FIG. 6(C) and also may have a function of resetting thepotentials supplied to the capacitor 116 and the liquid crystal element117.

FIGS. 8(A) to 8(C) each illustrate a specific example of a wiring forsupplying “V_(ref)” illustrated in FIG. 2 or the like. As illustrated inFIG. 8(A), the wiring 128 can be used as the wiring for supplying“V_(ref)” in the case where a light-emitting element is used as thedisplay element. Since “V_(ref)” is preferably 0 V, GND, or a lowpotential, the wiring 128 also has a function of supplying at least anyof these potentials. To the wiring 128, “V_(ref)” is supplied at thetiming when data is written to the node NP, and a high potential poweris supplied at the timing when the light-emitting element 114 emitslight. Alternatively, as illustrated in FIG. 8(B), the wiring 129 thatsupplies a low potential may be used as the wiring for supplying“V_(ref)”.

In the case where a liquid crystal element is used as the displayelement as illustrated in FIG. 8(C), the wiring 133 can be used as thewiring for supplying “V_(ref)”. Alternatively, the wiring 134 may beused. Note that regardless of the kind of the display element, adedicated common wiring for supplying “V_(ref)” may be provided.

In one embodiment of the present invention, as illustrated in examplesof FIGS. 9(A) and 9(B), a structure in which transistors are providedwith back gates may be employed. FIG. 9(A) shows a structure in whichthe back gates are electrically connected to the front gates, which hasan effect of increasing on-state currents. FIG. 9(B) illustrates astructure in which the back gates are electrically connected to a wiring135 capable of supplying a constant potential, which enables control ofthe threshold voltages of the transistors. The transistors included inthe circuit block 110 may also have back gates.

Next, simulation results of a pixel operation are described. FIG. 10illustrates a structure including pixels (PIX) in one column and thecircuit 11 connected to the column, which are used in the simulation.The number of pixels is assumed to be 100 to 2000, and the circuit block110 has the structure (the liquid crystal element and the capacitor)shown in FIG. 7(A). Performed is the simulation of voltage changes ofthe nodes NP at the time when the number of pixels is changed.

Parameters used in the simulation are as follows: the transistor sizesare L/W=4 μm/30 μm (transistors Tr1, Tr2, and Tr3) and L/W=4 μm/10 μm(transistor Tr4), the capacitance value of a capacitor Cs is 100 fF, thecapacitance value of a liquid crystal element Clc is 100 fF, and commonelectrodes VCOM and TCOM are set to 0 V. In addition, a voltage appliedto a gate of a transistor is set to +15 V as “H” and −10 V as “L”.

Furthermore, as many resistors RI corresponding to parasitic resistanceand capacitors C3 corresponding to parasitic capacitance as the pixelsare incorporated in a wiring PL connecting the circuit 11 and the pixelsPIX. Since the capacitance of the capacitors C1 (the sum of C2), theparasitic capacitance of the wiring PL (the sum of C3), and theresistance of the wiring PL (the sum of RI) depend on the number ofpixels, values shown in Table 1 are used. Note that SPICE is used ascircuit simulation software.

TABLE 1 Capacitance value Parasitic capacitance Parasitic resistancePixel of capacitor C1 of wiring PL of wiring PL number (sum of C2) [F](sum of C3) [F] (sum of R1) [Ω] 100 2.0E−12 7.3E−13 453 500 1.0E−113.7E−12 2267 1000 2.0E−11 7.3E−12 4533 2000 4.0E−11 1.5E−11 9067

FIG. 11 is a timing chart used in the simulation. Here, all of theweights (W) and the data (D) are set to 5 V. Furthermore, “V_(ref)” isset to 0 V.

FIGS. 12(A) to 12(D) show simulation results when the number of pixelsare 100, 500, 1000, and 2000. In each of the drawings, changes involtage at the nodes NP when write operation is performed on the pixelsPIX[1] to PIX[4] are shown on the time scale.

In the case where the number of pixels is 100, stable writing can beperformed, but addition of data is not sufficiently performed becausethe total capacitance value of C1 is small.

In the case where the number of pixels is 500, it is shown that thesimulation parameters used in this time match and addition of data inaccordance with the capacity ratio is possible.

In the case where the number of pixels is 1000, owing to a relativelylarge total capacitance value of C1, writing of W is not completelyfinished in the pixel PIX[1] on which writing is initially performedwithin the writing time, and thus the data potential does notsufficiently rise. Wiring is stably performed on the pixel PIX[2] andthe following pixels owing to the remaining charge in the wiring PL.

In the case where the number of pixels is 2000, owing to an excessivetotal capacitance value of C1, writing of W is not finished in time inthe pixel PIX[1], and thus the voltage is low. The excessive totalcapacitance value of C1 influences the pixel PIX[2] and the followingpixels.

The above simulation results confirm that a combination of appropriateoperation parameters and an appropriate number of pixels enables stabledata addition.

Although the total capacitance value of the capacitor C1 stronglyinfluences the operation, the capacitance value of the capacitor C1 canbe easily adjusted by the number of capacitors C2 connected in parallel.Therefore, even when the number of pixels arranged in a column is 1000,2000, or more, an appropriate operation such as the one shown in FIG.12(B) can be performed by adjusting the number of capacitors C2.

Next, a simulation result of the pixel layout is described. FIG. 13 is adiagram corresponding to the pixels PIX[1] to PIX[3] shown in FIG. 10and showing an example of a layout of three pixels arranged in avertical direction.

Note that FIG. 13 shows the layout as far as pixel electrodes PEcorresponding to the nodes NP, and the capacitors Cs are omitted.Bottom-gate transistors (with back gates) are illustrated as an example.Note that the pixel pitch is assumed to be 136 m.

A conductive layer formed through the same process as that of a gatewiring and a conductive layer formed through the same process as that ofa source wiring are used as a pair of electrodes in the capacitor C1.The two conductive layers overlap with each other with an insulatinglayer (e.g., a gate insulating film) therebetween in a region of thedisplay region.

That is, one capacitor C2 is provided in each pixel. Furthermore, thecapacitors C2 are connected in parallel and thus are equivalent to onelarge capacitor.

That is, the capacitor C1 is divided and arranged; therefore, theaperture ratio and transmittance of the pixel can be improved. Note thata connection wiring BR that serves as a bridge between gate wirings ispreferably used for electrical connection between one of the conductivelayers of one capacitor and one of the conductive layers of anothercapacitor. The connection wiring BR can be formed through the sameprocess as that of the source wiring, for example. With a pixel pitch of136 μm, the aperture ratio ((area of pixel electrode PE)/(pixel area))is estimated at approximately 90.5%.

The above simulation results show the effect of one embodiment of thepresent invention.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments and the like, asappropriate.

Embodiment 2

In this embodiment, a structure example of a display device using aliquid crystal element and a structure example of a display device usinga light-emitting element are described. Note that the description of thecomponents, operations, and functions of the display device described inEmbodiment 1 is omitted in this embodiment.

FIGS. 14(A) to 14(C) are diagrams each illustrating a structure of adisplay device in which one embodiment of the present invention can beused.

The adder circuit and the pixel described in Embodiment 1 can be used inthe display device described in this embodiment. Note that a scan linedriver circuit and a signal line driver circuit which are describedbelow correspond to the gate driver and the source driver, respectively.

In FIG. 14(A), a sealant 4005 is provided to surround a display portion215 provided over a first substrate 4001, and the display portion 215 issealed with the sealant 4005 and a second substrate 4006.

In FIG. 14(A), a scan line driver circuit 221 a, a signal line drivercircuit 231 a, a signal line driver circuit 232 a, and a common linedriver circuit 241 a each include a plurality of integrated circuits4042 provided over a printed circuit board 4041. The integrated circuits4042 are each formed using a single crystal semiconductor or apolycrystalline semiconductor. The common line driver circuit 241 a hasa function of supplying a prescribed potential to the wirings 128, 129,132, 133, 135, and the like described in Embodiment 1.

Signals and potentials are supplied to the scan line driver circuit 221a, the common line driver circuit 241 a, the signal line driver circuit231 a, and the signal line driver circuit 232 a through an FPC (Flexibleprinted circuit) 4018.

The integrated circuits 4042 included in the scan line driver circuit221 a and the common line driver circuit 241 a each have a function ofsupplying a selection signal to the display portion 215. The integratedcircuits 4042 included in the signal line driver circuit 231 a and thesignal line driver circuit 232 a each have a function of supplying imagedata to the display portion 215. The integrated circuits 4042 aremounted in a region different from the region surrounded by the sealant4005 over the first substrate 4001.

Note that the connection method of the integrated circuits 4042 is notparticularly limited; a wire bonding method, a COG (Chip On Glass)method, a TCP (Tape Carrier Package) method, a COF (Chip On Film)method, or the like can be used.

FIG. 14(B) illustrates an example in which the integrated circuits 4042included in the signal line driver circuit 231 a and the signal linedriver circuit 232 a are mounted by a COG method. Some or all of thedriver circuits can be formed over the same substrate as the displayportion 215, whereby a system-on-panel can be formed.

In the example illustrated in FIG. 14(B), the scan line driver circuit221 a and the common line driver circuit 241 a are formed over the samesubstrate as the display portion 215. When the driver circuits areformed concurrently with pixel circuits in the display portion 215, thenumber of components can be reduced. Accordingly, the productivity canbe increased.

In FIG. 14(B), the sealant 4005 is provided to surround the displayportion 215, the scan line driver circuit 221 a, and the common linedriver circuit 241 a provided over the first substrate 4001. The secondsubstrate 4006 is provided over the display portion 215, the scan linedriver circuit 221 a, and the common line driver circuit 241 a.Consequently, the display portion 215, the scan line driver circuit 221a, and the common line driver circuit 241 a are sealed together withdisplay elements with the use of the first substrate 4001, the sealant4005, and the second substrate 4006.

Although the signal line driver circuit 231 a and the signal line drivercircuit 232 a are separately formed and mounted on the first substrate4001 in the example illustrated in FIG. 14(B), one embodiment of thepresent invention is not limited to this structure. The scan line drivercircuit may be separately formed and then mounted, part of the signalline driver circuits or part of the scan line driver circuits may beseparately formed and then mounted. The signal line driver circuit 231 aand the signal line driver circuit 232 a may be formed over the samesubstrate as the display portion 215, as illustrated in FIG. 14(C).

In some cases, the display device encompasses a panel in which thedisplay element is sealed, and a module in which an IC or the likeincluding a controller is mounted on the panel.

The display portion and the scan line driver circuit provided over thefirst substrate each include a plurality of transistors. As thetransistors, the transistor described in the above embodiment can beused.

Transistors included in a peripheral driver circuit and transistorsincluded in the pixel circuits of the display portion may have the samestructure or different structures. The transistors included in theperipheral driver circuit may be transistors having the same structure,or transistors having two or more kinds of structures may be included.Similarly, the transistors included in the pixel circuits may betransistors having the same structure, or transistors having two or morekinds of structures may be included.

An input device 4200 can be provided over the second substrate 4006. Thedisplay devices illustrated in FIGS. 14(A) to 14(C) and provided withthe input device 4200 can function as a touch panel.

There is no particular limitation on a sensor element included in thetouch panel of one embodiment of the present invention. A variety ofsensors capable of sensing an approach or a contact of a sensing targetsuch as a finger or a stylus can be used as the sensor element.

For example, a variety of types such as a capacitive type, a resistivetype, a surface acoustic wave type, an infrared type, an optical type,and a pressure-sensitive type can be used for the sensor.

In this embodiment, a touch panel including a capacitive sensor elementis described as an example.

Examples of the capacitive sensor element include a surface capacitivesensor element and a projected capacitive sensor element. Examples ofthe projected capacitive sensor element include a self-capacitive sensorelement and a mutual capacitive sensor element. The use of a mutualcapacitive sensor element is preferred because multiple points can besensed simultaneously.

The touch panel of one embodiment of the present invention can have anyof a variety of structures, including a structure in which a displaydevice and a sensor element that are separately formed are attached toeach other and a structure in which an electrode and the like includedin a sensor element are provided on one or both of a substratesupporting a display element and a counter substrate.

FIGS. 15(A) and 15(B) illustrate an example of the touch panel. FIG.15(A) is a perspective view of a touch panel 4210. FIG. 15(B) is aschematic perspective view of the input device 4200. Note that forclarity, only typical components are shown.

The touch panel 4210 has a structure in which a display device and asensor element that are separately formed are attached to each other.

The touch panel 4210 includes the input device 4200 and the displaydevice, which are provided to overlap with each other.

The input device 4200 includes a substrate 4263, an electrode 4227, anelectrode 4228, a plurality of wirings 4237, a plurality of wirings4238, and a plurality of wirings 4239. For example, the electrode 4227can be electrically connected to the wiring 4237 or the wiring 4239. Inaddition, the electrode 4228 can be electrically connected to the wiring4239. An FPC 4272 b is electrically connected to each of the pluralityof wirings 4237 and the plurality of wirings 4238. An IC 4273 b can beprovided for the FPC 4272 b.

Alternatively, a touch sensor may be provided between the firstsubstrate 4001 and the second substrate 4006 in the display device. Inthe case where a touch sensor is provided between the first substrate4001 and the second substrate 4006, either a capacitive touch sensor oran optical touch sensor including a photoelectric conversion element maybe used.

FIGS. 16(A) and 16(B) are cross-sectional views of a portion indicatedby chain line N1-N2 in FIG. 14(B). Display devices illustrated in FIGS.16(A) and 16(B) each include an electrode 4015, and the electrode 4015is electrically connected to a terminal included in the FPC 4018 throughan anisotropic conductive layer 4019. In FIGS. 16(A) and 16(B), theelectrode 4015 is electrically connected to a wiring 4014 in an openingformed in an insulating layer 4112, an insulating layer 4111, and aninsulating layer 4110.

The electrode 4015 is formed of the same conductive layer as a firstelectrode layer 4030, and the wiring 4014 is formed of the sameconductive layer as source electrodes and drain electrodes of atransistor 4010 and a transistor 4011.

The display portion 215 and the scan line driver circuit 221 a providedover the first substrate 4001 each include a plurality of transistors.In FIGS. 16(A) and 16(B), the transistor 4010 included in the displayportion 215 and the transistor 4011 included in the scan line drivercircuit 221 a are shown as an example. Note that in the examplesillustrated in FIGS. 16(A) and 16(B), the transistor 4010 and thetransistor 4011 are bottom-gate transistors but may be top-gatetransistors.

In FIGS. 16(A) and 16(B), the insulating layer 4112 is provided over thetransistor 4010 and the transistor 4011. In FIG. 16(B), a partition wall4510 is formed over the insulating layer 4112.

The transistor 4010 and the transistor 4011 are provided over aninsulating layer 4102. The transistor 4010 and the transistor 4011 eachinclude an electrode 4017 formed over the insulating layer 4111. Theelectrode 4017 can serve as a back gate electrode.

The display devices illustrated in FIGS. 16(A) and 16(B) each include acapacitor 4020. The capacitor 4020 includes an electrode 4021 formed inthe same step as a gate electrode of the transistor 4010, and anelectrode formed in the same step as the source electrode and the drainelectrode. The electrodes overlap with each other with an insulatinglayer 4103 therebetween.

In general, the capacitance of a capacitor provided in a pixel portionof a display device is set in consideration of the leakage current orthe like of transistors provided in the pixel portion so that chargescan be held for a predetermined period. The capacitance of the capacitoris set in consideration of the off-state current of the transistors orthe like.

The transistor 4010 provided in the display portion 215 is electricallyconnected to the display element. FIG. 16(A) illustrates an example of aliquid crystal display device using a liquid crystal element as thedisplay element. In FIG. 16(A), a liquid crystal element 4013 serving asthe display element includes the first electrode layer 4030, a secondelectrode layer 4031, and a liquid crystal layer 4008. Note that aninsulating layer 4032 and an insulating layer 4033 functioning asalignment films are provided so that the liquid crystal layer 4008 ispositioned therebetween. The second electrode layer 4031 is provided onthe second substrate 4006 side, and the first electrode layer 4030 andthe second electrode layer 4031 overlap with each other with the liquidcrystal layer 4008 therebetween.

A liquid crystal element having a variety of modes can be used as theliquid crystal element 4013. For example, a liquid crystal element usinga VA (Vertical Alignment) mode, a TN (Twisted Nematic) mode, an IPS(In-Plane-Switching) mode, an ASM (Axially Symmetric aligned Micro-cell)mode, an OCB (Optically Compensated Bend) mode, an FLC (FerroelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,an ECB (Electrically Controlled Birefringence) mode, a VA-IPS mode, aguest-host mode, or the like can be used.

As the liquid crystal display device described in this embodiment, anormally black liquid crystal display device such as a transmissiveliquid crystal display device employing a vertical alignment (VA) modemay be used. As the vertical alignment mode, an MVA (Multi-DomainVertical Alignment) mode, a PVA (Patterned Vertical Alignment) mode, anASV (Advanced Super View) mode, and the like can be used.

Note that the liquid crystal element is an element that controlstransmission and non-transmission of light by the optical modulationaction of liquid crystal. The optical modulation action of the liquidcrystal is controlled by an electric field applied to the liquid crystal(including a horizontal electric field, a vertical electric field, andan oblique electric field). As the liquid crystal used for the liquidcrystal element, thermotropic liquid crystal, low-molecular liquidcrystal, high-molecular liquid crystal, polymer dispersed liquid crystal(PDLC), ferroelectric liquid crystal, anti-ferroelectric liquid crystal,or the like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Although an example of a liquid crystal display device including aliquid crystal element with a vertical electric field mode isillustrated in FIG. 16, one embodiment of the present invention can beapplied to a liquid crystal display device including a liquid crystalelement with a horizontal electric field mode. In the case of employinga horizontal electric field mode, liquid crystal exhibiting a blue phasefor which an alignment film is not used may be used. The blue phase isone of liquid crystal phases, which is generated just before acholesteric phase changes into an isotropic phase while the temperatureof a cholesteric liquid crystal is increased. Since the blue phaseappears only in a narrow temperature range, a liquid crystal compositionin which a chiral material of 5 weight % or more is mixed is used forthe liquid crystal layer 4008 in order to improve the temperature range.The liquid crystal composition that contains liquid crystal exhibiting ablue phase and a chiral material has a short response speed and exhibitsoptical isotropy. In addition, the liquid crystal composition containingliquid crystal exhibiting a blue phase and a chiral material does notneed alignment treatment and has small viewing angle dependence. Sincean alignment film does not need to be provided and rubbing treatment isunnecessary, electrostatic discharge damage caused by the rubbingtreatment can be prevented and defects or damage of the liquid crystaldisplay device in the manufacturing process can be reduced.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating layer and is provided in order to control a distance (a cellgap) between the first electrode layer 4030 and the second electrodelayer 4031. Note that a spherical spacer may alternatively be used.

A black matrix (a light-blocking layer); a coloring layer (a colorfilter); an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member; or the likemay be provided as appropriate if needed. For example, circularpolarization may be employed by using a polarizing substrate and aretardation substrate. In addition, a backlight, a side light, or thelike may be used as a light source. A micro LED or the like may be usedas the backlight or the side light.

In the display device illustrated in FIG. 16(A), a light-blocking layer4132, a coloring layer 4131, and an insulating layer 4133 are providedbetween the second substrate 4006 and the second electrode layer 4031.

Examples of a material that can be used for the light-blocking layerinclude carbon black, titanium black, a metal, a metal oxide, and acomposite oxide containing a solid solution of a plurality of metaloxides. The light-blocking layer may be a film containing a resinmaterial or may be a thin film of an inorganic material such as a metal.Stacked films containing the material of the coloring layer can also beused for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material of a coloring layer whichtransmits light of a certain color and a film containing a material of acoloring layer which transmits light of another color can be employed.It is preferable that the coloring layer and the light-blocking layer beformed using the same material because the same manufacturing apparatuscan be used and the process can be simplified.

Examples of a material that can be used for the coloring layer include ametal material, a resin material, and a resin material containing apigment or a dye. The light-blocking layer and the coloring layer can beformed by an inkjet method, for example.

The display devices shown in FIGS. 16(A) and 16(B) each include theinsulating layer 4111 and an insulating layer 4104. As the insulatinglayer 4111 and the insulating layer 4104, insulating layers throughwhich an impurity element does not easily pass are used. A semiconductorlayer of the transistor is positioned between the insulating layer 4111and the insulating layer 4104, whereby entry of impurities from theoutside can be prevented.

A light-emitting element can be used as the display element included inthe display device. As the light-emitting element, for example, an ELelement that utilizes electroluminescence can be used. An EL elementincludes a layer containing a light-emitting compound (also referred toas an “EL layer”) between a pair of electrodes. By generating apotential difference between the pair of electrodes that is greater thanthe threshold voltage of the EL element, holes are injected to the ELlayer from the anode side and electrons are injected to the EL layerfrom the cathode side. The injected electrons and holes are recombinedin the EL layer and a light-emitting substance contained in the EL layeremits light.

As the EL element, an organic EL element or an inorganic EL element canbe used, for example. Note that an LED (including a micro LED) that usesa compound semiconductor as a light-emitting material is one of ELelements, and the LED can also be used.

In the organic EL element, by voltage application, electrons areinjected from one electrode to the EL layer and holes are injected fromthe other electrode to the EL layer. The carriers (electrons and holes)are recombined, the light-emitting organic compound forms an excitedstate, and light is emitted when the excited state returns to a groundstate. Owing to such a mechanism, this light-emitting element isreferred to as a current-excitation light-emitting element.

Note that in addition to the light-emitting compound, the EL layer mayfurther include a substance with a high hole-injection property, asubstance with a high hole-transport property, a hole-blocking material,a substance with a high electron-transport property, a substance with ahigh electron-injection property, a substance with a bipolar property (asubstance with a high electron- and hole-transport property), or thelike.

The EL layer can be formed by a method such as an evaporation method(including a vacuum evaporation method), a transfer method, a printingmethod, an inkjet method, or a coating method.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element includes alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is positioned between dielectriclayers, which are further positioned between electrodes, and its lightemission mechanism is localization type light emission that utilizesinner-shell electron transition of metal ions. Note that the descriptionis made here using an organic EL element as the light-emitting element.

In order to extract light emitted from the light-emitting element, atleast one of the pair of electrodes needs to be transparent. Atransistor and a light-emitting element are formed over a substrate. Thelight-emitting element can have a top emission structure in which lightemission is extracted from the surface on the side opposite to thesubstrate; a bottom emission structure in which light emission isextracted from the surface on the substrate side; or a dual emissionstructure in which light emission is extracted from both surfaces. Thelight-emitting element having any of the emission structures can beused.

FIG. 16(B) shows an example of a light-emitting display device using alight-emitting element as a display element (also referred to as an “ELdisplay device”). A light-emitting element 4513 serving as the displayelement is electrically connected to the transistor 4010 provided in thedisplay portion 215. Note that the structure of the light-emittingelement 4513 is a stacked-layer structure of the first electrode layer4030, a light-emitting layer 4511, and the second electrode layer 4031;however, this embodiment is not limited to this structure. The structureof the light-emitting element 4513 can be changed as appropriatedepending on the direction in which light is extracted from thelight-emitting element 4513, or the like.

The partition wall 4510 is formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 4510 be formed using a photosensitive resin materialto have an opening portion over the first electrode layer 4030 such thata side surface of the opening portion slopes with continuous curvature.

The light-emitting layer 4511 may be formed using a single layer or aplurality of layers stacked.

The emission color of the light-emitting element 4513 can be white, red,green, blue, cyan, magenta, yellow, or the like depending on thematerial for the light-emitting layer 4511.

As a color display method, there are a method in which thelight-emitting element 4513 that emits white light is combined with acoloring layer and a method in which the light-emitting element 4513that emits light of a different emission color is provided in eachpixel. The former method is more productive than the latter method. Thelatter method, which requires separate formation of the light-emittinglayer 4511 pixel by pixel, is less productive than the former method.However, the latter method can provide higher color purity of theemission color than the former method. In the latter method, the colorpurity can be further increased when the light-emitting element 4513 hasa microcavity structure.

Note that the light-emitting layer 4511 may contain an inorganiccompound such as quantum dots. For example, when used for thelight-emitting layer, the quantum dots can function as a light-emittingmaterial.

A protective layer may be formed over the second electrode layer 4031and the partition wall 4510 in order to prevent entry of oxygen,hydrogen, moisture, carbon dioxide, or the like into the light-emittingelement 4513. For the protective layer, silicon nitride, silicon nitrideoxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminumnitride oxide, DLC (Diamond Like Carbon), or the like can be used. In aspace enclosed by the first substrate 4001, the second substrate 4006,and the sealant 4005, a filler 4514 is provided for sealing. It ispreferable that the light-emitting element be packaged (sealed) with aprotective film (such as a laminate film or an ultraviolet curable resinfilm) or a cover member with high air-tightness and littledegasification in this manner so that the light-emitting element is notexposed to the outside air.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon; PVC(polyvinyl chloride), an acrylic-based resin, polyimide, an epoxy-basedresin, a silicone-based resin, PVB (polyvinyl butyral), EVA (ethylenevinyl acetate), or the like can be used. A drying agent may be containedin the filler 4514.

A glass material such as a glass frit or a resin material such as acurable resin that is curable at room temperature, such as atwo-component-mixture-type resin, a light curable resin, or athermosetting resin can be used for the sealant 4005. A drying agent maybe contained in the sealant 4005.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate on an emission surface ofthe light-emitting element. Furthermore, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on a surface so as to reduce theglare can be performed.

When the light-emitting element has a microcavity structure, light withhigh color purity can be extracted. Furthermore, when a microcavitystructure and a color filter are used in combination, the glare can bereduced and visibility of a displayed image can be increased.

The first electrode layer and the second electrode layer (also called apixel electrode layer, a common electrode layer, a counter electrodelayer, or the like) for applying voltage to the display element eachhave a light-transmitting property or a light-reflecting property, whichdepends on the direction in which light is extracted, the position wherethe electrode layer is provided, and the pattern structure of theelectrode layer.

Each of the first electrode layer 4030 and the second electrode layer4031 can be formed using a light-transmitting conductive material suchas indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tinoxide, indium tin oxide containing titanium oxide, indium zinc oxide, orindium tin oxide to which silicon oxide is added.

Each of the first electrode layer 4030 and the second electrode layer4031 can also be formed using one or more kinds selected from a metalsuch as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf),vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co),nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu),or silver (Ag); an alloy thereof; and a metal nitride thereof.

A conductive composition containing a conductive high molecule (alsoreferred to as conductive polymer) can be used for the first electrodelayer 4030 and the second electrode layer 4031. As the conductive highmolecule, a it-electron conjugated conductive high molecule can be used.For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, and acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof can be given.

Since the transistor is easily broken by static electricity or the like,a protective circuit for protecting the driver circuit is preferablyprovided. The protective circuit is preferably formed using a nonlinearelement.

Note that as illustrated in FIG. 17, a stacked structure including aregion where a transistor and a capacitor overlap with each other in theheight direction may be employed. For example, when the transistor 4011and a transistor 4022 included in the driver circuit are provided tooverlap with each other, a display device with a narrow frame can beprovided. Furthermore, when the transistor 4010, a transistor 4023, thecapacitor 4020, and the like included in the pixel circuit are providedto at least partly overlap with each other, the aperture ratio and theresolution can be improved. Although an example in which the stackedstructure is employed for the liquid crystal display device illustratedin FIG. 16(A) is illustrated in FIG. 17, the stacked structure may beemployed for the EL display device illustrated in FIG. 16(B).

In addition, a light-transmitting conductive film with highvisible-light-transmitting property is used as an electrode or a wiringin the pixel circuit, whereby transmittance of light in the pixel can beincreased and the aperture ratio can be substantially improved. Notethat in the case where an OS transistor is used, a semiconductor layeralso has a light-transmitting property and thus the aperture ratio canbe further increased. These are effective even when transistors and thelike are not stacked.

The display device may have a structure with a combination of a liquidcrystal display device and a light-emitting device.

The light-emitting device is disposed on the side opposite to thedisplay surface or on an end portion of the display surface. Thelight-emitting device has a function of supplying light to the displayelement. The light-emitting device can also be referred to as abacklight.

Here, the light-emitting device can include a plate-like or sheet-likelight guide portion (also referred to as a light guide plate) and aplurality of light-emitting elements which emit light of differentcolors. When the light-emitting elements are disposed in the vicinity ofthe side surface of the light guide portion, light can be emitted fromthe side surface of the light guide portion to the inside. The lightguide portion has a mechanism that changes an optical path (alsoreferred to as a light extraction mechanism), and this enables thelight-emitting device to emit light uniformly to a pixel portion of adisplay panel. Alternatively, the light-emitting device may be provideddirectly under the pixel without providing the light guide portion.

The light-emitting device preferably includes light-emitting elements ofthree colors, red (R), green (G), and blue (B). In addition, alight-emitting element of white (W) may be included. A light emittingdiode (LED) is preferably used as these light-emitting elements.

Furthermore, the light-emitting elements preferably have extremely highcolor purities; the full width at half maximum (FWHM) of the emissionspectrum of the light-emitting element is less than or equal to 50 nm,preferably less than or equal to 40 nm, further preferably less than orequal to 30 nm, still further preferably less than or equal to 20 nm.Note that the full width at half maximum of the emission spectrum ispreferably as small as possible, and can be, for example, greater thanor equal to 1 nm. Thus, when color display is performed, a vivid imagewith high color reproducibility can be displayed.

As the red light-emitting element, an element whose wavelength of anemission spectrum peak is in a range from 625 nm to 650 nm is preferablyused. As the green light-emitting element, an element whose wavelengthof an emission spectrum peak is in a range from 515 nm to 540 nm ispreferably used. As the blue light-emitting element, an element whosewavelength of an emission spectrum peak is in a range from 445 nm to 470nm is preferably used.

The display device can make the light-emitting elements of the threecolors blink sequentially, drive the pixels in synchronization withthese light-emitting elements, and perform color display on the basis ofthe successive additive color mixing method. This driving method canalso be referred to as a field-sequential driving.

By the field-sequential driving, a clear color image can be displayed.In addition, a smooth moving image can be displayed. When theabove-described driving method is used, one pixel does not need to beformed with subpixels of different colors, which can make an effectivereflection area (also referred to as an effective display area or anaperture ratio) per pixel large; thus, a bright image can be displayed.Furthermore, the pixels do not need to be provided with color filters,and thus can have improved transmittance and achieve brighter imagedisplay. In addition, the manufacturing process can be simplified, andthe manufacturing costs can be reduced.

FIGS. 18(A) and 18(B) each illustrate an example of a schematiccross-sectional view of a display device capable of the field-sequentialdriving. A backlight unit capable of emitting light of RGB colors isprovided on the first substrate 4001 side of the display device. Notethat in the field-sequential driving, the RGB colors are expressedthrough time division light emission, and thus color filters are notneeded.

A backlight unit 4340 a illustrated in FIG. 18(A) has a structure inwhich a plurality of light-emitting elements 4342 are provided directlyunder a pixel with a diffusing plate 4352 positioned therebetween. Thediffusing plate 4352 have functions of diffusing light emitted from thelight-emitting element 4342 to the first substrate 4001 side and makingthe luminance in a display portion uniform. Between the light-emittingelement 4342 and the diffusing plate 4352, a polarizing plate may beprovided if necessary. The diffusing plate 4352 is not necessarilyprovided if not needed. The light-blocking layer 4132 may be omitted.

The backlight unit 4340 a can include a large number of light-emittingelements 4342, which enables bright image display. Moreover, there areadvantages that a light guide plate is not needed and light efficiencyof the light-emitting element 4342 is less likely to be lowered. Notethat the light-emitting element 4342 may be provided with a lightdiffusion lens 4344 if necessary.

A backlight unit 4340 b illustrated in FIG. 18(B) has a structure inwhich a light guide plate 4341 is provided directly under a pixel withthe diffusing plate 4352 positioned therebetween. The plurality oflight-emitting elements 4342 are provided at an end portion of the lightguide plate 4341. The light guide plate 4341 has an uneven shape on theside opposite to the diffusing plate 4352, and can scatter waveguidedlight with the uneven shape to emit the light in the direction of thediffusing plate 4352.

The light-emitting element 4342 can be fixed to a printed circuit board4347. Note that in FIG. 18(B), the light-emitting elements 4342 of RGBcolors overlap with each other; however, the light-emitting elements4342 of RGB colors can be arranged to be lined up in the depthdirection. A reflective layer 4348 that reflects visible light may beprovided on the side surface of the light guide plate 4341 which isopposite to the light-emitting element 4342.

The backlight unit 4340 b can reduce the number of light-emittingelements 4342, leading to reductions in cost and thickness.

A light-scattering liquid crystal element may be used as the liquidcrystal element. The light-scattering liquid crystal element ispreferably an element containing a composite material of liquid crystaland a polymer molecule. For example, a polymer dispersed liquid crystalelement can be used. Alternatively, a polymer network liquid crystal(PNLC) element may be used.

The light-scattering liquid crystal element has a structure in which aliquid crystal portion is provided in a three-dimensional networkstructure of a resin portion sandwiched between a pair of electrodes. Asa material used in the liquid crystal portion, for example, a nematicliquid crystal can be used. A photocurable resin can be used for theresin portion. The photocurable resin can be a monofunctional monomer,such as acrylate or methacrylate; a polyfunctional monomer, such asdiacrylate, triacrylate, dimethacrylate, or trimethacrylate; or apolymerizable compound obtained by mixing these.

The light-scattering liquid crystal element performs display bytransmitting or scattering light utilizing the anisotropy of arefractive index of a liquid crystal material. The resin portion mayhave the anisotropy of a refractive index. When liquid crystal moleculesare arranged in a certain direction in accordance with a voltage appliedto the light-scattering liquid crystal element, a direction is generatedat which a difference in a refractive index between the liquid crystalportion and the resin portion is small. Incident light along thedirection passes without being scattered in the liquid crystal portion.Thus, the light-scattering liquid crystal element is perceived in atransparent state from the direction. In contrast, when liquid crystalmolecules are arranged randomly in accordance with the applied voltage,a large difference in refractive index between the liquid crystalportion and the resin portion is not generated, and incident light isscattered in the liquid crystal portion. Thus, the light-scatteringliquid crystal element is in an opaque state regardless of the viewingdirection.

FIG. 19(A) illustrates a structure in which the liquid crystal element4013 of the display device illustrated in FIG. 18(A) is replaced with alight-scattering liquid crystal element 4016. The light-scatteringliquid crystal element 4016 includes a composite layer 4009 including aliquid crystal portion and a resin portion, the first electrode layer4030, and the second electrode layer 4031. Although components relatingto the field-sequential driving are the same as those in FIG. 18(A),when the light-scattering liquid crystal element 4016 is used, analignment film and a polarizing plate are not necessary. Note that thespherical spacer 4035 is illustrated, but the spacer 4035 may have acolumnar shape.

FIG. 19(B) illustrates a structure in which the liquid crystal element4013 of the display device illustrated in FIG. 18(B) is replaced withthe light-scattering liquid crystal element 4016. In the structure ofFIG. 18(B), it is preferable that light be transmitted when a voltage isnot applied to the light-scattering liquid crystal element 4016, andlight be scattered when a voltage is applied. With such a structure, thedisplay device can be transparent in a normal state (without display).In that case, color display can be performed when light scatteringoperation is performed.

FIGS. 20(A) to 20(E) illustrate modification examples of the displaydevice in FIG. 19(B). Note that in FIGS. 20(A) to 20(E), some componentsin FIG. 19(B) are used and the other components are not illustrated forsimplicity.

FIG. 20(A) illustrates a structure in which the first substrate 4001 hasa function of a light guide plate. An uneven surface may be provided onan outer surface of the first substrate 4001. With this structure, alight guide plate does not need to be provided additionally, leading toa reduction in a manufacturing cost. Furthermore, the attenuation oflight caused by the light guide plate also does not occur; accordingly,light emitted from the light-emitting element 4342 can be efficientlyutilized.

FIG. 20(B) illustrates a structure in which light enters from thevicinity of an end portion of the composite layer 4009. By utilizingtotal reflection at the interface between the composite layer 4009 andthe second substrate 4006 and the interface between the composite layer4009 and the first substrate 4001, light can be emitted to the outsidefrom the light-scattering liquid crystal element. For the resin portionof the composite layer 4009, a material having a refractive index higherthan that of the first substrate 4001 and that of the second substrate4006 is used.

Note that the light-emitting element 4342 is not limited to be providedon one side of the display device, and may be provided on each of twosides facing each other as illustrated in FIG. 20(C). Furthermore, thelight-emitting elements 4342 may be provided on three sides or foursides. When the light-emitting elements 4342 are provided on a pluralityof sides, attenuation of light can be compensated for and application toa large-area display element is possible.

FIG. 20(D) illustrates a structure in which light emitted from thelight-emitting element 4342 is guided to the display device through amirror 4345. With this structure, light can be guided easily with acertain angle to the display device; thus, total reflection light can beobtained efficiently.

FIG. 20(E) illustrates a structure in which a layer 4003 and a layer4004 are stacked over the composite layer 4009. One of the layer 4003and the layer 4004 is a support such as a glass substrate, and the othercan be formed of an inorganic film, a coating film of an organic resin,a film, or the like. For the resin portion of the composite layer 4009,a material having a refractive index higher than that of the layer 4004is used. For the layer 4004, a material having a refractive index higherthan that of the layer 4003 is used.

A first interface is formed between the composite layer 4009 and thelayer 4004, and a second interface is formed between the layer 4004 andthe layer 4003. With this structure, light passing through without beingtotally reflected at the first interface is totally reflected at thesecond interface and can be returned to the composite layer 4009.Accordingly, light emitted from the light-emitting element 4342 can beefficiently utilized.

Note that the structures in FIG. 19(B) and FIGS. 20(A) to 20(E) can becombined with each other.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments and the like, asappropriate.

Embodiment 3

In this embodiment, examples of transistors which can be used as thetransistors described in the above embodiments are described withreference to drawings.

The display device of one embodiment of the present invention can befabricated using a transistor with any of various structures, such as abottom-gate transistor or a top-gate transistor. Therefore, a materialof a semiconductor layer or the structure of a transistor can be easilychanged depending on the existing production line.

[Bottom-Gate Transistor]

FIG. 21(A1) is a cross-sectional view of a channel-protective transistor810, which is a type of bottom-gate transistor, in the channel lengthdirection. In FIG. 21(A1), the transistor 810 is formed over a substrate771. The transistor 810 includes an electrode 746 over the substrate 771with an insulating layer 772 therebetween. The transistor 810 alsoincludes a semiconductor layer 742 over the electrode 746 with aninsulating layer 726 therebetween. The electrode 746 can function as agate electrode. The insulating layer 726 can function as a gateinsulating layer.

Furthermore, an insulating layer 741 is provided over a channelformation region in the semiconductor layer 742. Furthermore, anelectrode 744 a and an electrode 744 b are provided over the insulatinglayer 726 to be partly in contact with the semiconductor layer 742. Theelectrode 744 a can function as one of a source electrode and a drainelectrode. The electrode 744 b can function as the other of the sourceelectrode and the drain electrode. Part of the electrode 744 a and partof the electrode 744 b are formed over the insulating layer 741.

The insulating layer 741 can function as a channel protective layer.With the insulating layer 741 provided over the channel formationregion, the semiconductor layer 742 can be prevented from being exposedat the time of forming the electrode 744 a and the electrode 744 b.Thus, the channel formation region in the semiconductor layer 742 can beprevented from being etched at the time of forming the electrode 744 aand the electrode 744 b. According to one embodiment of the presentinvention, a transistor with favorable electrical characteristics can beprovided.

The transistor 810 includes an insulating layer 728 over the electrode744 a, the electrode 744 b, and the insulating layer 741 and alsoincludes an insulating layer 729 over the insulating layer 728.

In the case where an oxide semiconductor is used for the semiconductorlayer 742, a material capable of removing oxygen from part of thesemiconductor layer 742 to generate oxygen vacancies is preferably usedat least for portions of the electrode 744 a and the electrode 744 bwhich are in contact with the semiconductor layer 742. The carrierconcentration in the regions of the semiconductor layer 742 where oxygenvacancies are generated is increased, so that the regions become n-typeregions (n⁺ layers). Accordingly, the regions can function as a sourceregion and a drain region. When an oxide semiconductor is used for thesemiconductor layer 742, examples of the material capable of removingoxygen from the semiconductor layer 742 to generate oxygen vacanciesinclude tungsten and titanium.

Formation of the source region and the drain region in the semiconductorlayer 742 makes it possible to reduce contact resistance between thesemiconductor layer 742 and each of the electrode 744 a and theelectrode 744 b. Accordingly, the electrical characteristics of thetransistor, such as the field-effect mobility and the threshold voltage,can be improved.

In the case where a semiconductor such as silicon is used for thesemiconductor layer 742, a layer that functions as an n-typesemiconductor or a p-type semiconductor is preferably provided betweenthe semiconductor layer 742 and the electrode 744 a and between thesemiconductor layer 742 and the electrode 744 b. The layer thatfunctions as an n-type semiconductor or a p-type semiconductor canfunction as the source region or the drain region in the transistor.

The insulating layer 729 is preferably formed using a material that hasa function of preventing or reducing diffusion of impurities into thetransistor from the outside. Note that the insulating layer 729 can beomitted as necessary.

A transistor 811 illustrated in FIG. 21(A2) is different from thetransistor 810 in that an electrode 723 that can function as a back gateelectrode is provided over the insulating layer 729. The electrode 723can be formed using a material and a method similar to those for theelectrode 746.

In general, a back gate electrode is formed using a conductive layer andpositioned so that a channel formation region in a semiconductor layeris positioned between the gate electrode and the back gate electrode.Thus, the back gate electrode can function in a manner similar to thatof the gate electrode. The potential of the back gate electrode may bethe same as the potential of the gate electrode or may be a groundpotential (GND potential) or an arbitrary potential. When the potentialof the back gate electrode is changed independently of the potential ofthe gate electrode, the threshold voltage of the transistor can bechanged.

The electrode 746 and the electrode 723 can each function as a gateelectrode. Thus, the insulating layer 726, the insulating layer 728, andthe insulating layer 729 can each function as a gate insulating layer.Note that the electrode 723 may be provided between the insulating layer728 and the insulating layer 729.

Note that in the case where one of the electrode 746 and the electrode723 is referred to as a “gate electrode”, the other is referred to as a“back gate electrode”. For example, in the transistor 811, in the casewhere the electrode 723 is referred to as a “gate electrode”, theelectrode 746 is referred to as a “back gate electrode”. In the casewhere the electrode 723 is used as a “gate electrode”, the transistor811 can be regarded as a kind of top-gate transistor. One of theelectrode 746 and the electrode 723 may be referred to as a “first gateelectrode”, and the other may be referred to as a “second gateelectrode”.

By providing the electrode 746 and the electrode 723 with thesemiconductor layer 742 therebetween and setting the potential of theelectrode 746 equal to the potential of the electrode 723, a region ofthe semiconductor layer 742 through which carriers flow is enlarged inthe film thickness direction; thus, the number of transferred carriersis increased. As a result, the on-state current of the transistor 811 isincreased and the field-effect mobility is increased.

Therefore, the transistor 811 is a transistor having high on-statecurrent for its occupation area. That is, the occupation area of thetransistor 811 can be small for required on-state current. According toone embodiment of the present invention, the occupation area of atransistor can be reduced. Therefore, according to one embodiment of thepresent invention, a semiconductor device having a high degree ofintegration can be provided.

The gate electrode and the back gate electrode are formed usingconductive layers and thus each have a function of preventing anelectric field generated outside the transistor from affecting thesemiconductor layer in which the channel is formed (in particular, anelectric field blocking function against static electricity and thelike). Note that when the back gate electrode is formed larger than thesemiconductor layer such that the semiconductor layer is covered withthe back gate electrode, the electric field blocking function can beenhanced.

When the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented, and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

According to one embodiment of the present invention, a transistor withfavorable reliability can be provided. Moreover, a semiconductor devicewith favorable reliability can be provided.

FIG. 21(B1) is a cross-sectional view of a channel-protective transistor820, which has a structure different from FIG. 21(A1), in the channellength direction. The transistor 820 has substantially the samestructure as the transistor 810 but is different from the transistor 810in that the insulating layer 741 covers end portions of thesemiconductor layer 742. The semiconductor layer 742 is electricallyconnected to the electrode 744 a through an opening portion formed byselectively removing part of the insulating layer 741 that overlaps withthe semiconductor layer 742. The semiconductor layer 742 is electricallyconnected to the electrode 744 b through another opening portion formedby selectively removing part of the insulating layer 741 that overlapswith the semiconductor layer 742. A region of the insulating layer 729that overlaps with the channel formation region can function as achannel protective layer.

A transistor 821 illustrated in FIG. 21(B2) is different from thetransistor 820 in that the electrode 723 that can function as a backgate electrode is provided over the insulating layer 729.

With the insulating layer 741, the semiconductor layer 742 can beprevented from being exposed at the time of forming the electrode 744 aand the electrode 744 b. Thus, the semiconductor layer 742 can beprevented from being reduced in thickness at the time of forming theelectrode 744 a and the electrode 744 b.

The distance between the electrode 744 a and the electrode 746 and thedistance between the electrode 744 b and the electrode 746 are longer inthe transistor 820 and the transistor 821 than in the transistor 810 andthe transistor 811. Thus, the parasitic capacitance generated betweenthe electrode 744 a and the electrode 746 can be reduced. Moreover, theparasitic capacitance generated between the electrode 744 b and theelectrode 746 can be reduced. According to one embodiment of the presentinvention, a transistor with favorable electrical characteristics can beprovided.

FIG. 21(C1) is a cross-sectional view of a channel-etched transistor825, which is a type of bottom-gate transistor, in the channel lengthdirection. In the transistor 825, the electrode 744 a and the electrode744 b are formed without the insulating layer 741. Thus, part of thesemiconductor layer 742 that is exposed at the time of forming theelectrode 744 a and the electrode 744 b might be etched. However, sincethe insulating layer 741 is not provided, the productivity of thetransistor can be increased.

A transistor 826 illustrated in FIG. 21(C2) is different from thetransistor 825 in that the electrode 723 that can function as a backgate electrode is provided over the insulating layer 729.

FIGS. 22(A1) to 22(C2) are cross-sectional views of the transistors 810,811, 820, 821, 825, and 826 in the channel width direction,respectively.

In each of the structures illustrated in FIGS. 22(B2) and 22(C2), thegate electrode is connected to the back gate electrode, and the gateelectrode and the back gate electrode have the same potential. Inaddition, the semiconductor layer 742 is positioned between the gateelectrode and the back gate electrode.

The length of each of the gate electrode and the back gate electrode inthe channel width direction is longer than the length of thesemiconductor layer 742 in the channel width direction. In the channelwidth direction, the whole of the semiconductor layer 742 is coveredwith the gate electrode and the back gate electrode with the insulatinglayers 726, 741, 728, and 729 positioned therebetween.

In this structure, the semiconductor layer 742 included in thetransistor can be electrically surrounded by electric fields of the gateelectrode and the back gate electrode.

The transistor device structure in which the semiconductor layer 742 inwhich the channel formation region is formed is electrically surroundedby electric fields of the gate electrode and the back gate electrode, asin the transistor 821 or the transistor 826, can be referred to as aSurrounded channel (S-channel) structure.

With the S-channel structure, an electric field for inducing a channelcan be effectively applied to the semiconductor layer 742 by one or bothof the gate electrode and the back gate electrode, which improves thecurrent drive capability of the transistor and offers high on-statecurrent characteristics. In addition, the transistor can be miniaturizedbecause the on-state current can be increased. The S-channel structurecan also increase the mechanical strength of the transistor.

[Top-Gate Transistor]

A transistor 842 illustrated in FIG. 23(A1) is a type of top-gatetransistor. The electrode 744 a and the electrode 744 b are electricallyconnected to the semiconductor layer 742 through opening portions formedin the insulating layer 728 and the insulating layer 729.

Part of the insulating layer 726 that does not overlap with theelectrode 746 is removed, and an impurity is introduced into thesemiconductor layer 742 using the electrode 746 and the remaininginsulating layer 726 as masks, so that an impurity region can be formedin the semiconductor layer 742 in a self-aligned manner. The transistor842 includes a region where the insulating layer 726 extends beyond endportions of the electrode 746. The semiconductor layer 742 in a regioninto which the impurity is introduced through the insulating layer 726has a lower impurity concentration than the semiconductor layer 742 in aregion into which the impurity is introduced not through the insulatinglayer 726. Thus, an LDD (Lightly Doped Drain) region is formed in aregion of the semiconductor layer 742 which overlaps with the insulatinglayer 726 but does not overlap with the electrode 746.

A transistor 843 illustrated in FIG. 23(A2) is different from thetransistor 842 in that the electrode 723 is included. The transistor 843includes the electrode 723 that is formed over the substrate 771. Theelectrode 723 includes a region overlapping with the semiconductor layer742 with the insulating layer 772 therebetween. The electrode 723 canfunction as a back gate electrode.

As in a transistor 844 illustrated in FIG. 23(B1) and a transistor 845illustrated in FIG. 23(B2), the insulating layer 726 in a region thatdoes not overlap with the electrode 746 may be completely removed.Alternatively, as in a transistor 846 illustrated in FIG. 23(C1) and atransistor 847 illustrated in FIG. 23(C2), the insulating layer 726 maybe left.

Also in the transistor 842 to the transistor 847, after the formation ofthe electrode 746, an impurity is introduced into the semiconductorlayer 742 using the electrode 746 as a mask, so that an impurity regioncan be formed in the semiconductor layer 742 in a self-aligned manner.According to one embodiment of the present invention, a transistor withfavorable electrical characteristics can be provided. Furthermore,according to one embodiment of the present invention, a semiconductordevice having a high degree of integration can be provided.

FIGS. 24(A1) to 24(C2) are cross-sectional views of the transistors 842,843, 844, 845, 846, and 847 in the channel width direction,respectively.

The transistor 843, the transistor 845, and the transistor 847 each havethe above-described S-channel structure. However, one embodiment of thepresent invention is not limited to this, and the transistor 843, thetransistor 845, and the transistor 847 do not necessarily have theS-channel structure.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments and the like, asappropriate.

Embodiment 4

Examples of an electronic device that can use the display device of oneembodiment of the present invention include display devices, personalcomputers, image storage devices or image reproducing devices providedwith storage media, cellular phones, game machines including portablegame machines, portable data terminals, e-book readers, cameras such asvideo cameras and digital still cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (e.g.,car audio players and digital audio players), copiers, facsimiles,printers, multifunction printers, automated teller machines (ATM), andvending machines. FIG. 25 show specific examples of such electronicdevices.

FIG. 25(A) shows a digital camera, which includes a housing 961, ashutter button 962, a microphone 963, a speaker 967, a display portion965, operation keys 966, a zoom lever 968, a lens 969, and the like. Theuse of the display device of one embodiment of the present invention forthe display portion 965 enables display of a variety of images.

FIG. 25(B) shows a digital signage, which has large display portions922. The digital signage can be installed on the side surface of apillar 921, for example. The use of the display device of one embodimentof the present invention for the display portion 922 enables displaywith high display quality.

FIG. 25(C) shows an example of a cellular phone, which includes ahousing 951, a display portion 952, an operation button 953, an externalconnection port 954, a speaker 955, a microphone 956, a camera 957, andthe like. The display portion 952 of the cellular phone includes a touchsensor. Operations such as making a call and inputting text can beperformed by touch on the display portion 952 with a finger, a stylus,or the like. The housing 951 and the display portion 952 haveflexibility and can be used in a bent state as shown in the figure. Theuse of the display device of one embodiment of the present invention forthe display portion 952 enables display of a variety of images.

FIG. 25(D) shows a video camera, which includes a first housing 901, asecond housing 902, a display portion 903, an operation key 904, a lens905, a connection portion 906, a speaker 907, and the like. Theoperation key 904 and the lens 905 are provided on the first housing901, and the display portion 903 is provided on the second housing 902.The use of the display device of one embodiment of the present inventionfor the display portion 903 enables display of a variety of images.

FIG. 25(E) shows a television, which includes a housing 971, a displayportion 973, an operation key 974, speakers 975, a communicationconnection terminal 976, an optical sensor 977, and the like. Thedisplay portion 973 includes a touch sensor that enables inputoperation. The use of the display device of one embodiment of thepresent invention for the display portion 973 enables display of avariety of images.

FIG. 25(F) shows a portable data terminal, which includes a housing 911,a display portion 912, speakers 913, a camera 919, and the like. A touchpanel included in the display portion 912 enables input and output ofinformation. The use of the display device of one embodiment of thepresent invention for the display portion 912 enables display of avariety of images.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments and the like, asappropriate.

REFERENCE NUMERALS

10: pixel, 11: circuit, 11 a: circuit, 11 b: circuit, 12: source driver,12 a: source driver, 12 b: source driver, 13: gate driver, 15: displayregion, 16: selection circuit, 101: transistor, 102: transistor, 103:transistor, 104: capacitor, 105: transistor, 106: capacitor, 110:circuit block, 111: transistor, 112: transistor, 113: capacitor, 114:light-emitting element, 115: transistor, 116: capacitor, 117: liquidcrystal element, 118: transistor, 119: transistor, 120: circuit, 121:wiring, 121 a: wiring, 121 b: wiring, 122: wiring, 122 a: wiring, 122 b:wiring, 123: wiring, 124: wiring, 125: wiring, 127: wiring, 128: wiring,129: wiring, 130: wiring, 131: wiring, 132: wiring, 133: wiring, 134:wiring, 135: wiring, 136: pixel pitch, 138: wiring, 215: displayportion, 221 a: scan line driver circuit, 231 a: signal line drivercircuit, 232 a: signal line driver circuit, 241 a: common line drivercircuit, 723: electrode, 724 a: electrode, 724 b: electrode, 726:insulating layer, 728: insulating layer, 729: insulating layer, 741:insulating layer, 742: semiconductor layer, 744 a: electrode, 744 b:electrode, 746: electrode, 771: substrate, 772: insulating layer, 810:transistor, 811: transistor, 820: transistor, 821: transistor, 825:transistor, 826: transistor, 842: transistor, 843: transistor, 844:transistor, 845: transistor, 846: transistor, 847: transistor, 901:housing, 902: housing, 903: display portion, 904: operation key, 905:lens, 906: connection portion, 907: speaker, 911: housing, 912: displayportion, 913: speaker, 919: camera, 921: pillar, 922: display portion,951: housing, 952: display portion, 953: operation button, 954: externalconnection port, 955: speaker, 956: microphone, 957: camera, 961:housing, 962: shutter button, 963: microphone, 965: display portion,966: operation key, 967: speaker, 968: zoom lever, 969: lens, 971:housing, 973: display portion, 974: operation key, 975: speaker, 976:communication connection terminal, 977: optical sensor, 4001: substrate,4003: layer, 4004: layer, 4005: sealant, 4006: substrate, 4008: liquidcrystal layer, 4009: composite layer, 4010: transistor, 4011:transistor, 4013: liquid crystal element, 4014: wiring, 4015: electrode,4016: light-scattering liquid crystal element, 4017: electrode, 4018:FPC, 4019: anisotropic conductive layer, 4020: capacitor, 4021:electrode, 4022: transistor, 4023: transistor, 4030: electrode layer,4031: electrode layer, 4032: insulating layer, 4033: insulating layer,4035: spacer, 4041: printed circuit board, 4042: integrated circuit,4102: insulating layer, 4103: insulating layer, 4104: insulating layer,4110: insulating layer, 4111: insulating layer, 4112: insulating layer,4131: coloring layer, 4132: light-blocking layer, 4133: insulatinglayer, 4200: input device, 4210: touch panel, 4227: electrode, 4228:electrode, 4237: wiring, 4238: wiring, 4239: wiring, 4263: substrate,4272 b: FPC, 4273 b: IC, 4340 a: backlight unit, 4340 b: backlight unit,4341: light guide plate, 4342: light-emitting element, 4344: lens, 4345:mirror, 4347: printed circuit board, 4348: reflective layer, 4352:diffusing plate, 4510: partition wall, 4511: light-emitting layer, 4513:light-emitting element, and 4514: filler.

The invention claimed is:
 1. A display device comprising: a firstcircuit comprising a first transistor, a second transistor, a thirdtransistor, and a first capacitor; and a pixel comprising a fourthtransistor, wherein the first transistor, the second transistor, and thethird transistor are positioned outside a display region, wherein thefirst capacitor and the pixel are positioned in the display region,wherein the first circuit is configured to generate third data by addingfirst data and second data, wherein the pixel is configured to performdisplay in accordance with the third data, wherein one of a source and adrain of the first transistor is electrically connected to one of asource and a drain of the fourth transistor and one electrode of thefirst capacitor, wherein the other electrode of the first capacitor iselectrically connected to one of a source and a drain of the secondtransistor and one of a source and a drain of the third transistor,wherein the other of the source and the drain of the first transistor iselectrically connected to the other of the source and the drain of thesecond transistor, and wherein the first capacitor comprises a pluralityof second capacitors which are electrically connected in parallel. 2.The display device according to claim 1, wherein the pixel comprises asecond circuit, wherein the second circuit comprises a display element,and wherein the other of the source and the drain of the fourthtransistor is electrically connected to the second circuit.
 3. Thedisplay device according to claim 1, wherein the pixel comprises asecond circuit, wherein the second circuit comprises a fifth transistor,a third capacitor, and a light-emitting element, wherein the other ofthe source and the drain of the fourth transistor is electricallyconnected to a gate of the fifth transistor and one electrode of thethird capacitor, and wherein one of a source and a drain of the fifthtransistor is electrically connected to the light-emitting element. 4.The display device according to claim 1, wherein the pixel comprises asecond circuit, wherein the second circuit comprises a fifth transistor,a third capacitor, a light-emitting element, and a sixth transistor,wherein the other of the source and the drain of the fourth transistoris electrically connected to a gate of the fifth transistor and oneelectrode of the third capacitor, wherein one of a source and a drain ofthe fifth transistor is electrically connected to one of a source and adrain of the sixth transistor, and wherein the other of the source andthe drain of the sixth transistor is electrically connected to thelight-emitting element.
 5. The display device according to claim 1,wherein the pixel comprises a liquid crystal element, and wherein theother of the source and the drain of the fourth transistor iselectrically connected to the liquid crystal element.
 6. The displaydevice according to claim 1, wherein the first transistor, the secondtransistor, the third transistor, and the fourth transistor eachcomprise a metal oxide in a channel formation region.
 7. The displaydevice according to claim 1, wherein a channel width of each of thefirst transistor, the second transistor, and the third transistor islarger than a channel width of the fourth transistor.
 8. An electronicdevice comprising the display device according to claim
 1. 9. A displaydevice comprising: a first circuit comprising a first transistor and afirst capacitor; and a pixel, wherein the first transistor is positionedoutside a display region, wherein the first capacitor and the pixel arepositioned in the display region, wherein the first circuit isconfigured to generate third data by adding first data and second data,wherein the pixel is configured to perform display in accordance withthe third data, and wherein the first capacitor comprises a plurality ofsecond capacitors which are electrically connected in parallel.
 10. Thedisplay device according to claim 9, wherein the pixel comprises alight-emitting element or a liquid crystal element.
 11. The displaydevice according to claim 9, wherein the first transistor comprises ametal oxide in a channel formation region.
 12. An electronic devicecomprising the display device according to claim 9.